In the oilfield, scale is a solid that precipitates out of solution or accumulates on the surface of subterranean materials, such as in fractures or subterranean flow pathways, or on equipment downhole, such as production tubing, gravel packing screens, or on equipment above the surface. Scale is a mineral or solid that is formed due to scale-forming ions that are present in water or petroleum downhole, such as Fe2+, Ca2+, Mg2+, Sr2+, and Ba2+. Examples of scale include carbonate salts, sulfate or sulfide salts, such as BaSO4, SrSO4, CaSO4, and CaCO3. Scale can decrease the permeability of a subterranean formation, reduce well productivity, and shorten the lifetime of production equipment. Cleaning and removal of scale generally requires stopping production and is both time-consuming and costly. Scale control challenges are a leading cause of declining production worldwide. Scale costs the petroleum industry millions of dollars each year in scale control and removal costs and in deferred production.
Hydraulic fracturing is an important technique that includes placing or extending channels from the wellbore to the reservoir. This operation includes hydraulically injecting a fracturing fluid into a wellbore penetrating or adjacent to a petroleum-producing subterranean formation and forcing the fracturing fluid against the surrounding subterranean material by pressure. The subterranean material is forced to crack, creating or enlarging one or more fractures. Proppant can be placed in fractures to prevent or reduce closure. The fractures can provide flow or can provide improved flow of the recoverable fluids from the formation, such as petroleum materials.
Viscous fluids are frequently used during downhole operations. For example, during a pad stage of fracturing, a fluid including viscosifer can be injected to begin to break into an area and initiate fractures to produce sufficient penetration and width to allow proppant-laden later stages to enter. In other examples, during various downhole operations solid materials are formed into a suspension with a viscous fluid and injected downhole. For example, during the slurry phase of hydraulic fracturing operations, a suspension of proppant is formed with fracturing fluid and injected downhole for deposition in fractures. Another example includes gravel pack operations, wherein a suspension of gravel is formed in a viscous fluid for transport downhole. Scale control can be valuable during operations including the injection of a suspension downhole. For example, during the slurry phase of hydraulic fracturing, as fractures are formed or widened, often water can be struck, causing infusion of water containing scale-forming ions into the fracture and wellbore and subsequent undesirable formation of scale.
Viscous fluids for downhole operations are often formed using a gel or crosslinked gel and a crosslinker that contains a metal or transition metal such as at least one of chromium, aluminum, antimony, zirconium, titanium, calcium, boron, iron, silicon, copper, zinc, magnesium, and an ion thereof. When scale inhibitors are exposed to crosslinkers, the scale inhibitor tends to chelate or bind to the metal or transition metal, which reduces or entirely eliminates the ability of the scale inhibitor to effectively inhibit scale. Therefore, current scale inhibitors are incompatible with viscous fluids including crosslinkers, making scale difficult to inhibit during various scale-prone downhole operations that include the use of viscous fluids.